To support transmission of a peak data rate up to 1 Gbits/s in a communication system, a carrier aggregation (CA) technology has been adopted as a method for expanding EPS bandwidth in a long term evolution (LTE) system at present. The main idea of the CA technology is to aggregate multiple component carriers (CC) into a carrier with a relatively larger bandwidth to support high-rate data transmission. FIG. 1 is a schematic diagram of a bandwidth structure adopted for transmitting data by adopting the CA technology in the related art. As shown in the figure, the downlink channel bandwidth for transmitting data is formed by aggregating five carriers of 20 MHz, including carrier 1, carrier 2, carrier 3, carrier 4 and carrier 5, and it could be understood that the aggregated carriers may be continuous or discontinuous on a frequency domain.
CA may be divided into two types, one of which is intra-band CA, and the other one of which is inter-band CA. For the intra-band CA, because multiple aggregated carriers are on the same frequency band, the coverage of data transmission of the carriers is consistent. For the inter-band CA, because the multiple aggregated carriers are on different frequency bands, when frequency ranges occupied by the frequency bands are relatively further, the coverage difference of data transmission of the carriers is relatively larger. Generally, the carrier coverage of low frequency ranges is large, and the carrier coverage of high frequency ranges is relatively small. As shown in FIG. 2 which is a schematic diagram of different carrier coverage under the inter-band CA in the related art, the blank area is a range covered by 800 Mhz carrier, the filled area is a range covered by 3 Ghz carrier, and it could be seen that the range covered by the 3 Ghz carrier is smaller than that covered by the 800 Mhz carrier.
It could be seen from FIG. 2 that, if a user equipment (UE) is in the center of a cell, the UE may simultaneously use a high-frequency carrier and a low-frequency carrier in the inter-band CA to transmit data, whereas if the UE moves to the margin of the cell, the UE cannot use the high-frequency carrier to transmit data, and the throughput of the data transmitted by the UE at the margin of the cell declines much compared with that by the UE in the center of the cell.
Accordingly, to improve the throughput of the data transmitted by the UE at the margin of the cell and enlarge the coverage of the high-frequency carriers, a relay node (RN) may be adopted at a network side of a cell to enlarge the coverage of the high-frequency carrier. FIG. 3 shows a schematic diagram of a network structure for enlarging the coverage of high-frequency carriers in the related art, wherein two RNs are added to continuously adopt the high-frequency carrier to transmit data, thus enlarging the coverage of the high-frequency carrier. However, even if the coverage of the high-frequency carrier is enlarged by adopting the RN, if the UE at the margin of the cell wants to simultaneously use the component carriers of high frequency range and low frequency range, the UE still needs to aggregate carriers from two different sites, namely the carrier of the low frequency range comes from a macro base station (DeNB, Donor eNodeB), and the carrier of the high frequency range comes from an RN. This herein is called as inter-site CA.
Therefore, it is a problem to be solved how the UE aggregates different carriers from the DeNB and the RN to transmit the data so as to improve the throughput of the data transmitted by the UE and enhance the experience of the UE.